Several different duplexing methods are used nowadays for channel access in point-to-multipoint networks such as cellular networks for dividing forward and reverse communication channels on the same physical communications medium. Examples of such duplexing methods include Time Division Duplex (TDD) and Frequency Division Duplex (FDD). TDD refers to duplex communication links where uplink is separated from downlink by the allocation of different time units in the same frequency band. TDD divides a data stream into frames and assigns different time units to forward and reverse transmissions, thereby allowing both types of transmissions to share the same transmission medium.
In consequence, there can be a conflict in TDD when uplink and downlink transmissions intend to access the transmission medium at the same or overlapping time unit(s) or time instance(s).
TDD is used, for example, in Long Term Evolution (LTE), and, more precisely, in Long-Term Evolution Time-Division Duplex (LTE-TDD) (also referred to as Time-division Long-Term Evolution (TD-LTE)). LTE-TDD is one of two variants of the LTE technology standard, the other being Frequency-Division Long-Term Evolution (LTE-FDD). LTE-TDD uses a single frequency, alternating between uploading and downloading data through time. Frequencies used for LTE-TDD range from 1850 MHz to 3800 MHz, with several different bands being used.
Up to now, the spectrum used by LTE is dedicated to LTE. This has the advantage that an LTE system does not need to care about coexistence with other non-3GPP (the abbreviation 3GPP normally stands for 3rd Generation Partnership Project) radio access technologies in the same spectrum and spectrum efficiency can be maximized. However, the spectrum allocated to LTE is limited which cannot meet the ever increasing demand for larger throughput from applications/services. Therefore, a new study item has been initiated in 3GPP on extending LTE to exploit unlicensed spectrum in addition to licensed spectrum.
The 3GPP initiative License Assisted Access (also sometimes referred to as Licensed Assisted Access and normally abbreviated as LAA) intends to allow LTE equipment to also operate in the unlicensed 5 GHz radio spectrum. In LAA, the unlicensed 5 GHz spectrum is used as a complement to the licensed spectrum so that throughput can be increased by utilizing radio resources from an LTE carrier in licensed as well as unlicensed spectrum. That is, aggregation of licensed and unlicensed radio resources for a wireless communication device can be implemented. Licensed radio resources can be considered to lie in a licensed band/licensed spectrum of the full frequency range that may be used for wireless communication. This whole spectrum is, in discrete bands, normally licensed to groups of similar services. Unlicensed radio resources may be considered to lie in an unlicensed band/spectrum of the full frequency range that may be used for wireless communication. Data aggregation in LAA can be achieved by carrier aggregation mechanisms.
Accordingly, devices can connect in the licensed spectrum to a primary cell and can use carrier aggregation to benefit from additional transmission capacity in the unlicensed spectrum when connected to a secondary cell. To reduce the changes required for aggregating licensed and unlicensed spectrum, the LTE frame timing in the primary cell is simultaneously used in the secondary cell.
Regulatory requirements may prohibit transmissions in the unlicensed spectrum without prior channel sensing. Since the unlicensed spectrum must be shared with other radios of similar or dissimilar wireless technologies, listen-before-talk (LBT) or other channel sensing techniques may be applied. Today, the unlicensed 5 GHz spectrum is mainly used by equipment implementing the IEEE 802.11 Wireless Local Area Network (WLAN) standard. This standard is known under its marketing brand “Wi-Fi.” In Europe, the LBT procedure is under the scope of EN 301.893 regulation. For LAA to operate in the 5 GHz spectrum, the LAA LBT procedure shall conform to requirements and minimum behaviors set forth in EN 301.893.
However, additional system designs and steps are needed to ensure coexistence of Wi-Fi and LAA with EN 301.893 LBT procedures.
U.S. Pat. No. 8,774,209 B2 relates to an apparatus and method for spectrum sharing using listen-before-talk with quiet periods, where LBT is adopted by frame-based orthogonal frequency-division multiplexing (OFDM) systems to determine whether the channel is free prior to transmission. A maximum transmission duration timer is used to limit the duration of a transmission burst, and is followed by a quiet period.
In TDD based wireless communication systems employing channel sensing mechanisms, there remains the problem of potential conflicts between Uplink and Downlink transmissions.